Vehicle exhaust emissions of regulatory concern include oxides of nitrogen. The oxides of nitrogen include, but are not limited to, nitric oxide, NO, and nitrogen dioxide, NO2. These compounds are frequently referred to as NOx as prescribed by the United States Environmental Protection Agency. Selective catalytic reduction (SCR) systems of NOx by nitrogen compounds, such as ammonia or urea, had been developed for various industrial applications. The application of SCR for mobile diesel engines requires overcoming several problems, including without limitation NOx reduction, NOx conversion at a wide temperature range, ammonia storage and ammonia slip. Ammonia emissions from SCR systems are known as ammonia slip. The ammonia slip increases at higher NH3/NOx ratios. SCR technology is effective in reducing diesel NOx emissions to levels required by a number of future emission standards, but systems and process improvements are a need.